Primary pulmonary alveolar proteinosis (PAP) is characterized by progressive accumulation of surfactant in the lungs resulting in respiratory failure as well as increased pulmonary and systemic infections that account for 18% of attributable mortality. Abrogation of GM-CSF signaling appears to be central to disease pathogenesis because GM-CSF knockout (GM-/-) mice develop PAP and have increased mortality from infection and because PAP in humans is associated with high levels of neutralizing anti-GM-CSF antibodies. We reported that GM-CSF is required to stimulate the terminal differentiation of alveolar macrophages (AMs) in mice and likely also in humans, and does so primarily via the hematopoietic transcription factor, PU.1. Neutrophil and monocyte counts are normal in PAP patients and GM-/- mice, suggesting GM-CSF has a critical role in the lung but not in hematopoiesis. Our Preliminary Data now show that low levels of anti-GM-CSF antibodies are present in disease-free, healthy individuals, and correlate inversely with neutrophil function;and that neutrophils are functionally impaired in both GMA/A mice and PAP patients. This proposal seeks to test the following general hypothesis: GM-CSF has a critical systemic role in innate immunity, stimulating mechanisms in myeloid cells determining basal levels of antimicrobial and other functions. Specifically, we hypothesize that (1) high levels of anti-GM-CSF antibodies are the cause of the clinical manifestations in PAP and not an epiphenomenon or a consequence of intercurrent microbial infection;and (2) low levels of anti-GM-CSF antibodies may play an important physiological role by binding and inactivating circulating GM-CSF, thereby modulating the basal immune responsiveness of myeloid cells. In Aim 1, we will determine the mechanism(s) by which anti-GM-CSF antibodies regulate functions in myeloid cells, including AMs, monocytes and neutrophils. In Aim 2, PAP will be recapitulated in healthy subjects by transfer of anti-GM-CSF antibodies from PAP patients into non-human primates, satisfying Koch's 2nd &3rd postulates. In Aim 3, we will utilize a novel immune model of PAP in mice to determine the critical threshold level of anti-GM-CSF antibodies that abrogate GM-CSF bioactivity in vivo and determine the kinetics and pharmacodynamics of myeloid cell dysfunction and onset and resolution of PAP. Expected results will establish that anti-GM-CSF antibodies cause the clinical manifestations in PAP patients, and will determine underlying mechanisms of myeloid cell dysfunction. Results have biological implications for the role of GM-CSF in mucosal barrier function beyond PAP in both health and disease. Clinical implications exist for PAP therapies, and also for anti-GM-CSF antibody-based therapies to treat serious inflammatory disorders, an approach for which significant commercial interest and development have now emerged.